The invention relates generally to providing an apparatus and process for electrolytic removal of material from products made from metallic alloys. More particularly, the invention relates to electrolytic removal of material from medical devices made of metallic alloys, and even more particularly, the invention relates to an apparatus and process for electrolytic removal of material from stents formed from a metallic alloy. The invention includes an apparatus and electrolytic solution, the process of electrolytic removal of material from a metallic stent using the apparatus, and a coil-link stent selectively treated using the apparatus and solutions.
Stents are generally metallic tube shaped intravascular devices which are placed within a blood vessel to structurally hold open the vessel. The device can be used to maintain the patency of a blood vessel immediately after intravascular treatments and can be used to reduce the likelihood of development of restenosis. Expandable stents are frequently used as they may travel in compressed form to the stenotic site generally either crimped onto an inflation balloon or compressed into a containment sheath in a known manner.
Expandable stents formed from metal offer a number of advantages and are widely used. Metallic, serpentine-shaped stents, for example, not only provide strength and rigidity once implanted, they also are designed to be sufficiently compressible and flexible for traveling through the tortuous pathways of the vessel (or artery) prior to arrival at the stenotic site.
It is highly desirable for the surface of the stent to be extremely smooth so that it can be inserted easily and experience low-friction travel through the tortuous vessel pathway prior to implantation. A roughened outer surface may result in increased frictional obstruction during insertion and excess drag during travel to the stenotic site as well as damaging the endothelium lining the vessel wall. A rough surface may cause frictional resistence to such an extent as to prevent travel to desired distal locations. A rough finish may also cause damage to the underlying inflation balloon. A less rough finish decreases thrombogenicity and increases corrosion resistance.
Stents have been formed from various metals including stainless steel, tantalum, titanium, platinum, nickel-titanium which is commonly called nitinol, and alloys formed with cobalt-chromium. Stainless steel has been extensively used to form stents and has often been the material of choice for stent construction. Stainless steel is corrosion resistant, strong, yet may be cut into very thin walled stent patterns.
Cobalt-chromium alloy is a metal that has proven advantages when used in stent applications. Stents made from cobalt-chromium alloy may be thinner and lighter in weight than stents made from other metallic materials, including stainless steel. Cobalt-chromium alloy is also a denser metal than stainless steel. Additionally, cobalt-chromium stents are nontranslucent to certain electromagnetic radiation waves, such as X-rays, and, relative to stainless steel stents, provide a higher degree of radiopacity, thus being easier to identify in the body under fluoroscopy.
Metal stents, however, suffer from a number disadvantages. They often require processing to eliminate undesirable burrs, nicks, or sharp ends. Expandable metal stents are frequently formed by use of a laser to cut a framework design from a tube of metal. The tubular stent wall is formed into a lattice arrangement consisting of metal struts with gaps therebetween. Laser cutting, however, typically is at high temperature and often leaves debris and slag material attached to the stent. Such material, if left on a stent, would render the stent unacceptable for implantation. Treatment to remove the slag, burrs, and nicks is therefore required to provide a device suitable for use in a body lumen.
Descaling is typically a first treatment of the surface in preparation for further surface treatment such as electropolishing. Descaling may include, for example, scraping the stent with a diamond file, followed by dipping the stent in hydrochloric acid or a hydrochloric acid mixture, and thereafter cleaning the stent ultrasonically. A successfully descaled metal stent should be substantially slag-free in preparation for subsequent electropolishing.
Further finishing is often accomplished by the well known technique of electropolishing. Grinding, vibration, and tumbling techniques are often not suited to be employed on small detailed parts such as stents.
Electropolishing and etching are electrochemical processes by which surface metal is dissolved. Sometimes referred to as “reverse plating,” the electropolishing process actually removes metal from the surface desired to be smoothed. The metal stent is connected to a power supply (the anode) and is immersed in a liquid electrolytic solution along with a metal cathode connected to the negative terminal of the power supply. Current is applied and flows from the stent, causing it to become polarized. The applied current is controlled to control the rate at which the metal ions of the anodic stent are generally removed and diffused through the solution to the cathode.
The rate of the electrochemical reaction is proportional to the current density. The positioning and thickness of the cathode in relation to the stent is important to make available an even distribution of current to the desired portion of the stent sought to be etched.
The straightforward application of current, however, does not necessarily translate to even distribution of current across the entire surface sought to be etched. One important feature to creating an even surface on the desired portion of the part is the formation of current differential during the electropolishing process across the surface. Electropolishing provides varied current density to the surface imperfections such as undulations creating protrusions and valleys on the surface. Current density is highest at high points on the surface and lowest at the low points. The increased current density at the raised points causes the metal to dissolve faster at these points thus leveling the surface while forming a corrosion-inhibiting oxide layer.
What is needed is an apparatus and a process for treating a product or device made of a metallic alloy to selectively remove a portion of a medical device, such as a stent, without removing metal from the remaining portion of the stent. The present invention satisfies these needs.